kbuild: Use targets := to tell kbuild about additional targets

kbuild: Use targets := to tell kbuild about additional targets

EXTRA_TARGETS served several purposes before:
1) List targets to be build (built-in only)
2) Inform kbuild of targets within the makefile

Mixing the above are causing confusion, so for now on
targets :=
is used to inform kbuild about targets in a Makefile - that it otherwise
did not know about.

kbuild uses the "targets :=" information to:
a) read the dependency file .target.cmd
b) delete the target file during make clean